Customers of lighting are increasingly choosing low-power lighting to meet their lighting needs. Typically, such low-power lighting employs halogen light bulbs, light-emitting diode light bulbs, compact fluorescent light bulbs, or other types of light bulbs or lamp assemblies that produce light with intensity on par with that of traditional incandescent light bulbs, but with significantly less power consumption. However, many of such light bulbs or lamp assemblies operate using a voltage (e.g., 12 volts) much less than that typically provided by traditional power infrastructures. For example, in the United States, public utilities generally provide electricity in the form of a 60-Hertz sinusoid with a magnitude of 120 volts. Thus, for low-power lamp assemblies to properly operate in the existing power infrastructure, a voltage transformer must be interposed between the public electricity source and the lamp assembly.
Transformers present in a power infrastructure may include magnetic or electronic transformers. A magnetic transformer typically comprises two coils of conductive material (e.g., copper) each wrapped around a core of material having a high magnetic permeability (e.g., iron) such that magnetic flux passes through both coils. In operation, an electric current in the first coil may produce a changing magnetic field in the core, such that the changing magnetic field induces a voltage across the ends of the secondary winding via electromagnetic induction. Thus, a magnetic transformer may step voltage levels up or down while providing electrical isolation in a circuit between components coupled to the primary winding and components coupled to the secondary winding.
On the other hand, an electronic transformer is a device which behaves in the same manner as a conventional magnetic transformer in that it steps voltage levels up or down while providing isolation and can accommodate load current of any power factor. An electronic transformer generally includes power switches which convert a low-frequency (e.g., direct current to 400 Hertz) voltage wave to a high-frequency voltage wave (e.g., in the order of 10,000 Hertz). A comparatively small magnetic transformer may be coupled to such power switches and thus provides the voltage level transformation and isolation functions of the conventional magnetic transformer.
FIG. 1 depicts a lighting system 101 that includes an electronic transformer 122 and a lamp assembly 142. Such a system may be used, for example, to transform a high voltage (e.g., 110V, 220 V) to a low voltage (e.g., 12 V) for use with a halogen lamp (e.g., an MR16 halogen lamp). In some instances, such a transformer 122 may be present in a lighting fixture configured to receive a lamp assembly 142, wherein such lamp assembly 142 includes a source of light (e.g., LEDs 152) for providing illumination. Generally, a transformer 122 designed to receive an incandescent or halogen lamp assembly “expects” a linear load (e.g., one which has a primarily constant impedance, in which current varies in a linear fashion with the voltage applied to the load). However, when a lamp that has a non-linear operating mode (e.g., including but not limited to a light-emitting diode, or LED, lamp) is used with an electronic transformer designed to receive a linear load, the electronic transformer may not function properly, due to the fact that the non-linear load may present widely varying impedances for different time durations.
Furthermore, an electronic transformer 122 may have a power rating range, such as from a minimum power rating to a maximum power rating (e.g., zero watts to 60 watts). When a non-linear load is coupled to electronic transformer 122, the varying non-linear impedance may consume power that falls outside the power rating range.
Referring to FIG. 1, lighting system 101 may receive an AC supply voltage VSUPPLY from voltage supply 104. The supply voltage VSUPPLY is, for example, a nominally 60 Hz/110 V line voltage in the United States of America or a nominally 50 Hz/220 V line voltage in Europe. Electronic transformer 122 may receive the AC supply voltage VSUPPLY at its input where it is rectified by a full-bridge rectifier formed by diodes 124. As voltage VSUPPLY increases in magnitude, voltage on capacitor 126 may increase to a point where diac 128 will turn on (the diac break-over voltage), thus also turning on transistor 129. Once transistor 129 is on, capacitor 126 may be discharged and oscillation will start due to the self-resonance of switching transformer 130, which includes a primary winding (T2a) and two secondary windings (T2b and T2c). Switching transformer 130 may be a saturable core transformer, and if the impedance of lamp assembly 142 is too low, the core of switching transformer 130 may saturate causing the voltage across the base-emitter junction of transistor 129 to go to zero, thus turning off transistor 129. Thus, the load presented to transformer 122 by lamp 142 must be low enough that the current through switching transformer 130 at the break-over voltage of diac 128 will saturate switching transformer 130, causing it to oscillate.
Lamp assembly 142 may receive the AC supply voltage VS at its input where it is rectified by a full-bridge rectifier formed by diodes 144. Such voltage may charge a capacitor 146, thus providing a direct current voltage VDD for power converter 148. Power converter 148 may be operable to provide a regulated voltage VLINK to LED driver 150, which itself may include circuitry for driving an output voltage or current to LEDs 152, thus generating photonic energy. During start-up of electronic transformer 122, capacitor 146 needs to charge to a voltage VDD sufficient to allow power converter 148 and LED driver 150 to begin steady-state operation. Because capacitor 146 is the primary load to electronic transformer 122 while power converter 148 and LED driver 150 start-up, a non-linear load is provided to electronic transformer 122. Upon start-up of electronic transformer 122, capacitor 146 initially provides a low impedance to electronic transformer 122, and electronic transformer 122 may begin oscillating. However, when capacitor 146 reaches a voltage VDD equal to a diode threshold voltage below the peak voltage value from electronic transformer 122, capacitor 146 then presents a high impedance to electronic transformer 122, and electronic transformer 122 may stop oscillation. If the voltage VDD across capacitor 146 is less than the steady-state voltage of power converter 148, then lamp assembly 142 may fail to present a linear load to electronic transformer 122.